Broiler breeders, the parental stock of meat producing chickens, are feed restricted to control their high growth rate. The feed restriction leads to poor welfare and stress.

High growth rate leads to:

• obesity 
• lameness 
• reduced fertility
• cardiovascular diseases 

Feed restricted broiler breeders experience:

• chronic hunger
• abnormal behaviours
• physiological stress

Common feeding regimens in the world:

• daily – reduced amount of food every day
• 1:1 skip-a-day – alternate days with feeding and fasting
• 5:2 skip-a-day – 5 non-consecutive days with feed, 2 non-consecutive days without feed.

Stress is the physiological response to a stressor, e.g.:

• temperature
• high density
• exposure to novel situations
• deprivation of water or food

The fight-or-flight response

The activation of the hypothalamus is an adaptive response to a stressor and it leads to increased blood pressure and heart rate by the secretion of epinephrine from the adrenal medulla which is the first reaction of stress. This stress response is commonly termed as the fight-or-flight response when energy is needed quickly.

Corticosterone

A later response is when the activation of the hypothalamo-pituitary-adrenal (HPA)-axis leads to a secretion of glucocorticoids from the adrenal cortex, such as corticosterone (CORT) in birds (Sapolsky et al., 2000). The secretion of CORT leads to a down regulation of the HPA axis, and inhibits the stress response (Sapolsky et al., 2000).

CORT increases plasma glucose concentration and suppresses the immune system when the secretion of CORT is prolonged.

The suppression of the immune system is a problem during chronic stress. During chronic stress, the number of lymphocytes are down-regulated and the number of granular leukocytes such as heterophils increases, which influences the inflammatory response (Sapolsky et al., 2000; Gross and Siegel, 1983).

Heterophil to lymphocyte (H/L) ratio

A low number of lymphocytes leads to a high H/L ratio and is interpreted as a high stress response. In comparison to plasma CORT, H/L ratios are altered slowly and are considered to be a less variable measurement of stress (Gross and Siegel, 1983).

Tonic immobility (TI)

The tonic immobility (TI) test is considered to be a measure of fear (Jones and Faure, 1981). The degree of feed restriction has been reported to reduce fearfulness in broiler breeders (Hocking et al., 2001; Savory et al., 1993).

Novel object (NO)

Another way to measure fear in chickens is to present a novel object (Jones, 1996, cited by Skinner-Noble et al., 2003). Fearfulness may be reduced with the severity of feed restriction (Skinner-Noble et al., 2003).